Backpack vacuum cleaner



FIG. 1 is a rear, left perspective view of a backpack vacuum cleaner showing our new design;

FIG. 2 is a front view thereof;

FIG. 3 is a right side view thereof;

FIG. 4 is a rear view thereof;

FIG. 5 is a left side view thereof; and,

FIG. 6 is a top view thereof.

The details shown in broken lines form no part of the claimed design. 

CLAIM The ornamental design for a backpack vacuum cleaner, as shown and described. 